Lone Pair Effect Research Articles

Effect of Lone Pairs on Molecular Resonance Energy.

In an effort to understand the theoretical parameters of charge-shift bonding, computational experiments have been designed to elucidate the factors effecting molecular resonance energy. Valence bond theory calculations have been used to calculate resonance energies of homonuclear bonds in the series [HnX-XHn]Z, where n = 0-3 and Z = 2n - 6, 2n - 4, 2n - 2, and 2n for X = C, N, O, and F, respectively. It is shown that the resonance energy decreases as the number of lone pairs increases. Calculated orbital contraction coefficients show that this unexpected result is due to the dominance of orbital size over the lone-pair effect. These results are irrespective of the HXX bond angles. It is also shown that the resonance energy increases with decreasing HXX bond angles due to the increase in p character in the bonding orbital.

Structure, properties, and disorder in the new distorted-Hollandite PbIr4Se8

The synthesis and physical properties of the new distorted-Hollandite PbIr4Se8 are reported. Powder X-ray diffraction and transmission electron microscopy show that the structure consists of edge- and corner-sharing IrSe6 octahedra, with one-dimensional channels occupied by Pb. The structure contains Se-Se anion-anion bonding, leading to an electron count of Pb2+(Ir3+)4(Se2)2-(Se2−)6, confirmed by bond-valence sums and diamagnetic behavior. Structural and heat capacity measurements demonstrate disorder on the Pb site, due to the combination of lone-pair effects and the large size of the one-dimensional channels. Comparisons are made to known Hollandite and pseudo-Hollandite structures, which demonstrates that the anion-anion bonding in PbIr4Se8 distorts its structure, to accommodate the Ir3+ state. An electronic structure calculation indicates semiconductor character with a band gap of 0.76(11)eV.

Adjacent Lone Pair (ALP) Effect: A Computational Approach for Its Origin.

The adjacent lone pair (ALP) effect is an experimental phenomenon in certain nitrogenous heterocyclic systems exhibiting the preference of the products with lone pairs separated over other isomers with lone pairs adjacent. A theoretical elucidation of the ALP effect requires the decomposition of intramolecular energy terms and the isolation of lone pair-lone pair interactions. Here we used the block-localized wavefunction (BLW) method within the ab initio valence bond (VB) theory to derive the strictly localized orbitals which are used to accommodate one-atom centered lone pairs and two-atom centered σ or π bonds. As such, interactions among electron pairs can be directly derived. Two-electron integrals between adjacent lone pairs do not support the view that the lone pair-lone pair repulsion is responsible for the ALP effect. Instead, the disabling of π conjugation greatly diminishes the ALP effect, indicating that the reduction of π conjugation in deprotonated forms with two σ lone pairs adjacent is one of the major causes for the ALP effect. Further electrostatic potential analysis and intramolecular energy decomposition confirm that the other key factor is the favorable electrostatic attraction within the isomers with lone pairs separated.

Adjacent Lone Pair (ALP) Effect: A Computational Approach for Its Origin

AbstractInvited for the cover of this issue are the teams led by Wei Wu (Xiamen University) and Yirong Mo (University of Western Michigan). The image depicts how π electron pairs (white figures) are more stable when the σ lone pairs (red figures) are separated. Read the full text of the article at 10.1002/chem.201600509.

Cover Picture: Adjacent Lone Pair (ALP) Effect: A Computational Approach for Its Origin (Chem. Eur. J. 22/2016)

Cover Picture: Adjacent Lone Pair (ALP) Effect: A Computational Approach for Its Origin (Chem. Eur. J. 22/2016)

Synthesis, crystal structure, and properties of a perovskite-related bismuth phase, (NH4)3Bi2I9

Organic-inorganic halide perovskites, especially methylammonium lead halide, have recently led to remarkable advances in photovoltaic devices. However, due to environmental and stability concerns around the use of lead, research into lead-free perovskite structures has been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, is prepared from solution and the structure solved by single crystal X-ray diffraction. The band gap, which is estimated to be 2.04 eV using UV-visible spectroscopy, is lower than that of CH3NH3PbBr3. The energy-minimized structure obtained from first principles calculations is in excellent agreement with the X-ray results and establishes the locations of the hydrogen atoms. The calculations also point to a significant lone pair effect on the bismuth ion. Single crystal and powder conductivity measurements are performed to examine the potential application of (NH4)3Bi2I9 as an alternative to the lead containing perovskites.

Drastic Deprotonation Reactivity Difference of 3- and 5-Alkylpyrazole Isomers, Their I2-Catalyzed Thermal Isomerization, and Telescoping Synthesis of 3,5-Dialkylpyrazoles: The "Adjacent Lone Pair Effect" Demystified.

N-Protected 3-alkylpyrazoles are easily deprotonated by (n)BuLi at the 5-position of the aromatic ring, while the 5-alkyl isomers are completely unreactive under the same conditions. Using computational analysis, we reveal that electron pair repulsion within the deprotonated anion is not the reason behind the lack of reactivity of 5-alkylpyrazoles. Instead, diminished π-resonance and attractive electrostatic interactions within the pyrazole ring are responsible for the observed effect. A greener, telescoping alternative to the synthesis of 3,5-dialkylpyrazoles is presented.

Synthesis and Structure of Three New Oxychalcogenides: A2O2Bi2Se3 (A = Sr, Ba) and Sr2O2Sb2Se3

Three new compounds have been added to the alkali chalcogenide and oxychalcogenide families: Sr2O2Bi2Se3, Ba2O2Bi2Se3, and Sr2O2Sb2Se3 were synthesized by direct combination of SrO or BaO with Bi2Se3 or Sb2Se3. The structure, determined from laboratory X-ray powder diffraction data, consists of double chains of edge-sharing BiSe4O square pyramids. Temperature-dependent resistance data reveal all three compounds to be insulators, while heat capacity data of Ba2O2Bi2Se3 and Sr2O2Bi2Se3, in conjunction with the literature reports, reveal low energy phonon modes due to bismuth lone pair effects. We propose a specific materials design principle connecting electron count to structural dimensionality by comparison to related chalcogenides, including the BiS2 superconductors.

Fluorite-related one-dimensional units in natural bismuth oxysulfates: the crystal structures of Bi14O16(SO4)5 and Bi30O33(SO4)9(AsO4)2.

The crystal structures of two new natural Bi oxysulfates with the formula Bi14O16(SO4)5 [labelled new phase I; monoclinic, space group C2, a = 21.658 (4), b = 5.6648 (9), c = 15.092 (3) Å, β = 119.433 (11)° and Z = 2] and Bi30O33(SO4)9(AsO4)2 [labelled new phase II; triclinic, space group P1, a = 5.670 (3), b = 13.9408 (9), c = 22.7908 (18) Å, α = 80.903 (5), β = 82.854 (14), γ = 78.27 (2)° and Z = 1] from the high-temperature fumarole deposit of the La Fossa crater at Vulcano (Aeolian Islands, Italy) are reported. The structures are built up by a combination of fluorite-related Bi-O units and isolated (SO4)(2-) tetrahedra (new phase I) or both (SO4)(2-) and (AsO4)(3-) tetrahedra (new phase II). Owing to the effect of stereoactive lone pairs of Bi(3+), Bi-O units in both the structures can be suitably described in terms of oxo-centered OBi4 tetrahedra. The structure of Bi14O16(SO4)5 is based upon one-dimensional [O16Bi14](10+) ribbons formed by six chains of edge-sharing OBi4 tetrahedra extending along [010]. In the structure of Bi30O33(SO4)9(AsO4)2 the same ribbon type coexists with another one-dimensional ribbon formed by seven chains of edge-sharing OBi4 tetrahedra and with the composition [O17Bi16](14+). Ribbons of the same type are joined by (SO4)(2-) and (AsO4)(3-) tetrahedra along [010] – if a reduced triclinic unit-cell setting is considered – so forming two different (001) slabs which alternate to each other along [001] and are joined by additional (SO4)(2-) tetrahedra. New phase I represents the natural analogues of synthetic Bi14O16(SO4)5, but with an ordered structure model.

Synthesis, crystal structures and properties of lead phosphite compounds

Here, we report the preparation and characterization of two lead(II) phosphites, namely, Pb2(HPO3)2 and Pb2(HPO3)(NO3)2 through hydrothermal reaction or simple solution synthesis, respectively. A new lead phosphite, namely, Pb2(HPO3)2, crystallizes in the noncentrosymmetric space group Cmc21 (no. 36), which features 3D framework formed by the interconnection of 2D layer of lead(II) phosphites and 1D chain of [Pb(HPO3)5]∞. The nonlinear optical properties of Pb2(HPO3)(NO3)2 have been studied for the first time. The synergistic effect of the stereo-active lone-pairs on Pb2+ cations and π-conjugated NO3 units in Pb2(HPO3)(NO3)2 produces a moderate second harmonic generation (SHG) response of ∼1.8×KDP (KH2PO4), which is phase matchable (type I). IR, UV–vis spectra and thermogravimetric analysis (TGA) for the two compounds were also measured.

A facile synthetic route to a new SHG material with two types of parallel π-conjugated planar triangular units.

A new SHG material, namely, Pb2(BO3)(NO3), which contains parallel π-conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO3)2 and Mg(BO2)2⋅H2O as starting materials. Its structure contains honeycomb [Pb2(BO3)]∞ layers with noncoordination [NO3](-) anions located at the interlayer space. Pb2(BO3)(NO3) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase-matchable. The large SHG coefficient of Pb2(BO3)(NO3) arises from the synergistic effect of the stereoactive lone pairs on Pb(2+) cations and parallel alignment of π-conjugated BO3 and NO3 units. Based on its unique properties, Pb2(BO3)(NO3) may have great potential as a high performance NLO material in photonic applications.

Enhanced photocatalytic activities of Bi2WO6 by introducing Zn to replace Bi lattice sites: a first-principles study

Zn doping can enhance the stereochemically active Bi lone pair effect at the top of the valence bands of Bi2WO6, which will lead to the decreasing of its band gap and the increasing of the density of electrons in its valence band maximum.

The High-pressure Thallium Triborate HP-TlB3O5

Abstract The thallium triborate HP-TlB3O5 (HP=high-pressure) was synthesized in a Walker-type multianvil apparatus under high-pressure=high-temperature conditions of 6 GPa and 1400°C. A mixture of thallium carbonate Tl2CO3 and boric acid H3BO3 initially heated at 850°C under ambient-pressure conditions was used as a precursor for the high-pressure experiment. Single-crystal X-ray diffraction data revealed that HP-TlB3O5 is isotypic to HP-M′B3O5 (M′ =K, Rb). Furthermore the B-O network is identical to the substitutional variants HP-M″B3O5 [M″ =Cs1-x(H3O)x (x=0.5 - 0.7), NH4]. HP-TlB3O5 crystallizes with eight formula units (Z =8) in the monoclinic space group C2=c (no. 15). The lattice parameters are a=996.3(2), b=884.0(2), c=913.1(2) pm, β =103:3(1)°, and V =782.5(3) Å3. Trigonal-planar BO3 groups, corner-sharing BO4 tetrahedra, and B2O6 groups consisting of two edge-sharing BO4 tetrahedra are present in the structure, forming a three-dimensional network. The thallium ions are located in channels of the boron-oxygen network being tenfold coordinated by oxygen atoms and do not show any lone pair effect at all. IR and Raman spectroscopic investigations were performed on single crystals of the compound.

Ga2SbCl7O – A Molecular Gallium Antimony Chloride Oxide Synthesized from a GaCl3 Melt

Abstract Gallium antimony chloride oxide, Ga2SbCl7O, is formed in a GaCl3‐based melt containing tellurium, antimony, SbCl3, and Sb2O3 in form of colorless, highly hygroscopic crystals. The crystal structure consists of discrete molecules, in which a bent SbOCl unit is coordinated by two GaCl3 groups. The Ga atoms coordinate the oxygen atom, resulting in a planar [Ga2SbO] central building unit. A distinct stereochemical lone pair effect is present at the antimony atom. In the crystal, the molecules are packed in the motif of a hexagonal closest packing. DFT calculations and NBO considerations show that the low lying molecular orbital comprising the lone pair at the oxygen atom emerges from a pure p orbital showing almost no delocalization and no steric effect.

Comparative study of A-site order in the lead-free bismuth titanates M1/2Bi1/2TiO3 (M=Li, Na, K, Rb, Cs, Ag, Tl) from first-principles

We investigate the possibility of enhancing chemical order in the relaxor ferroelectric Na1/2Bi1/2TiO3 upon substitution of Na+ by other monovalent cations M+ using total energy calculations based on density functional theory. All chemically available monovalent cations M+, which are Li, Na, Ag, K, Tl, Rb and Cs, are considered and an analysis of the structurally relaxed structures in terms of symmetry-adapted distortion modes is given in order to quantify the chemically induced structural distortions. We demonstrate that the replacement of Na+ by other monovalent cations can hardly alter the tendency of chemical order with respect to Na1/2Bi1/2TiO3. Only Tl1/2Bi1/2TiO3 and Ag1/2Bi1/2TiO3 show enhanced tendency for chemical ordering. Both heavy metals behave similar to the light alkali metals in terms of structural relaxations and relative stabilities of the ordered configurations. Although a comparison of the Goldschmidt factors of components (M TiO3)− reveals for Tl a value above the upper stability limit for perovskites, the additional lone-pair effect of Tl+ stabilizes the ordered structure.

Pb3B6O11F2: the first non-centrosymmetric lead borate fluoride with a large second harmonic generation response

The introduction of Pb2+ cations with the lone pair effect and the largest electronegativity of F− anions successfully results in a new non-centrosymmetric lead borate fluoride, Pb3B6O11F2, which possesses a large SHG response about four times that of KH2PO4 (KDP) and is type-I phase matchable. The structure is built from a 3-dimensional B–O network composed of hexaborate B6O14 groups and novel [FPb3]∞ layers. First-principles electronic structure calculations show that the calculated birefringence of Pb3B6O11F2 is about 0.07 in the visible region. Based on the measurement of the UV–Vis–NIR diffuse-reflectance spectrum, it exhibits a wide transparency range with a short UV cut-off edge at about 240 nm. All these properties indicate that Pb3B6O11F2 is a promising nonlinear optical crystal in the UV region.

DFT+U Study of Arsenate Adsorption on FeOOH Surfaces: Evidence for Competing Binding Mechanisms

On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites—directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1–As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2–As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1–As = 3.26–3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2–As = 3.31–3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with dFe1–As = 3.55–3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2–As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2–As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe–As distance. Our results show that the Fe–As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe–As distance cannot be used as a unique criterion to discriminate between adsorption modes.

Pb2B3O5.5(OH)2 and [Pb3(B3O7)](NO3): Facile Syntheses of New Lead(II) Borates by Simply Changing the pH Values of the Reaction Systems

Using lead metaborate as starting material, by only adjusting pH values of the reaction systems, a series of lead(II) borates were obtained in high yields. The new polar material, namely, Pb2B3O5.5(OH)2 (1), crystallizes in the noncentrosymmetric space group Pnn2 of the orthorhombic system. Its structure features a novel three-dimensional (3D) anionic network with large 14 member rings (MRs) tunnels composed of unique one-dimensional (1D) chains and dimeric B2O7 fragments, both of which are built up from solely BO4 tetrahedra, and the Pb(2+) cations are located at the above 14-MRs tunnels. The synergistic effect of the stereoactive lone-pairs on Pb(2+) cations in 1 produces a strong SHG response of ∼3× KDP (KH2PO4) which is type I phase-matchable. The first example of lead(II) borate nitrate, namely, [Pb3(B3O7)](NO3) (2), crystallizes in space group Pnma, and its structures features a 3D lead(II) borate cationic network structure in which (B3O7)(5-) anions are bridged by lead(II) cations, the nitrate anions are isolated, and located at the small voids of the cationic network. Thermal stability and optical properties as well as theoretical calculations based on density functional theory (DFT) methods were also performed.

ChemInform Abstract: Lone Pair Effect, Structural Distortions, and Potential for Superconductivity in Tl Perovskites.

AbstractPotential new superconductors of the type MTlX3 (M: Rb, Cs; X: F, Cl, Br) are characterized by DFT band structure calculations.

Lone Pair Effect, Structural Distortions, and Potential for Superconductivity in Tl Perovskites

Drawing the analogy to BaBiO3, we investigate via ab initio electronic structure calculations potential new superconductors of the type ATlX3 with A = Rb and Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. On the basis of chemical reasoning, supported by the calculations, we show that Tl-based perovskites have structural and charge instabilities driven by the lone pair effect, similar to the case of BaBiO3, effectively becoming A2Tl(+)Tl(3+)X6. We find that upon hole doping of RbTlCl3, structures without Tl(+) and Tl(3+) charge disproportionation become more stable, although the ideal cubic perovskite, often viewed as the best host for superconductivity, should not be the most stable phase in the system. The known superconductor (Sr,K)BiO3 and hole doped RbTlCl3, predicted to be most stable in the same tetragonal structure, display highly analogous calculated electronic band structures.